As shown in FIG. 12, a direct backlight device that is used as a backlight source in a liquid crystal display device is formed by arranging a fluorescent lamp 101 below a diffusion plate 103. In a general backlight device, a fluorescent film 102 having approximately the same thickness is formed over the inner surface of the tubular fluorescent lamp 101, and either light emitted from the fluorescent lamp 101 is directly directed to the diffusion plate 103 or light emitted to the opposite side from the diffusion plate 103 is reflected off a reflective plate 104 and is then directed to the diffusion plate 103.
However, in this general type of backlight device, the intensity of light that enters the diffusion plate 103 directly from the fluorescent lamp 101 is greatest at a portion which is closest to the fluorescent lamp 101 and which is directly above the fluorescent lamp 101, and as a portion is located further away from the portion directly above the fluorescent lamp 101, that is, the portion is further away from the fluorescent lamp 101, the light intensity is decreased. Although the portion away therefrom receives light reflected off the reflective plate 104, the light intensity varies on the light receiving side of the diffusion plate 103. Hence, as shown in FIG. 13, the brightness of illumination light varies due to those variations.
Hence, patent document 1 discloses the following: as shown in FIG. 14, a band-shaped fluorescent film 106 having a predetermined thickness is formed on the lower side of the inner surface of the fluorescent lamp 101 such that an angle formed by the fluorescent film is about 200 degrees; a band-shaped fluorescent film 107 having the same material and thickness as the one described above is formed in the middle portion of the upper surface side such that an angle formed by the fluorescent film is about 50 degrees; and thus, in an area, on the left and right of the fluorescent film 107 in the middle portion, in which an angle formed by the area is about 55 degrees, a pair of transparent light transmission window portions 108 having no fluorescent film is formed.
According to patent document 1, the intensity of light emitted from the fluorescent lamp 101 is greater in the middle portion of the upper surface side, where the fluorescent film 107 is formed, than in the window portions 108, but the distance traveled by the light that is emitted from the fluorescent lamp 101 and that reaches the diffusion plate 103 is greater from the middle portion of the upper surface side, where the fluorescent film 107 is formed, than from the window portions 108, with the result that it is possible to reduce variations in the intensity of light entering the diffusion plate 103.    Patent document 1: JP-A-H08-298100 (paragraph [0023] and FIG. 3)